The present disclosure is directed to adding a lip feature to a fan case flange shoulder bushing to allow for more efficient load distribution.
Gas turbine engine assemblies include an engine casing that extends around the turbine engine. Engine casings are fabricated from segmented sections that are coupled together via flanges extending from the sections of the casing. Adjacent flanges are coupled together with fasteners. The fasteners are inserted through flange bolt holes in parallel flanges extending perpendicularly outward from the casing section.
The flange bolt is typically made of a steel alloy. The steel bolt material is needed to withstand the forces of a break away fan blade or other fan component failure. The forces that impinge on the casing from a failed fan component are known as Fan Blade Out loads.
The casing material is typically an aluminum alloy. Thus the flange bolt material and the flange of the casing are dissimilar metals. Due to the dissimilar metals, corrosion is formed on the fan case flange bolt holes. This is due to the galvanic corrosion between the steel bolt and aluminum fan case. As the corrosion becomes more severe, the hole diameter becomes elongated. As a result, the bearing area between the bolt head and the flange hole is reduced.
The reduced bolt hole area results in insufficient parent material to withstand the Fan Blade Out load. Moreover, the reduced bearing area of the flange hole can be located near the fillet radius of the flange. Eventually, the corrosion reduces the bolt hole capability so it no longer meets the Fan Blade Out load. If no repair is conducted, the case can no longer be placed into service and will be scrapped at significant financial costs.
Rather than scraping the casing, a repair is conducted on the flange bolt hole. The flange bolt hole corrosion is removed and the bolt hole is enlarged. A bushing is placed in the bolt hole to receive the flange bolt. In order to redistribute the load to a bigger hole, a shoulder bushing design is used.
In order to have sufficient material to meet the requirements of the Fan Blade Out load, the thickness of the shoulder bushing is made larger, in some cases to a dimension of 0.070 inches. The bolt length is made longer in order to cater to the greater bushing thickness. Changing the bolt length requires new engineering design to ensure that the new bolt length meets the design loads. A formal Design Change including engineering time and cost is required to justify use of the longer bolt length. A Design Change creates added cost.